Ionic liquids are salts with melting points lower than 100° C. They may be used in a range of applications, including various chemical reactions, solvent processes, and electrochemistry. The use of ionic liquids as alkylation catalysts has attracted considerable attention in the field of petroleum refining.
Heretofore, several costly and time consuming wet chemical analyses were required in order to determine the composition of both fresh and used ionic liquids. Such tests are destructive and consume valuable ionic liquids, as well as having significant preparation time due to the number of tests performed and the sensitivity of the materials to ambient air.
Vibrational spectra of pure aluminum chloride-butylpyridinium chloride (bupyCl) melts having different AlCl3/bupyCl molar ratios are described by Gale et al. (Inorg. Chem. 1980, 19, 2240-2242). Subsequently, Tait et al. performed an IR study of ambient temperature chloroaluminates as a function of melt acidity; in the case of 1-methyl-3-ethylimidazolium chloride/aluminum chloride, results on the effects of adding water to acidic and basic melts were reported (Inorg. Chem. 1984, 23, 4352-4360). In both the above studies, vibrational modes were assigned to the corresponding molecular vibration but no attempt was made to develop correlations between signal intensity and melt concentration with respect to pyridinium salt, aluminum chloride or impurities. A method for monitoring ionic liquid catalyst deactivation by titrating hydrolyzed ionic liquid catalyst samples with a basic reagent was described in U.S. Patent Application Publication No. 20100129921 (Timken et al.), the disclosure of which is incorporated by reference herein in its entirety.
As noted above, sample preparation for wet chemistry analyses is both time consuming and may consume significant quantities of the ionic liquid. Accordingly, there is a need for a non-destructive, convenient, and efficient method for monitoring the composition of an ionic liquid before or during an ionic liquid catalyzed process.